This invention relates to electromagnetic circuit breakers in which current overload sensing is accomplished by a solenoid coil adapted to pull in a plunger when the overload current exceeds some predetermined value and thereby actuate an armature and associated electrical switch in circuit with the supply current. The plunger is spring biased outwardly of the coil and is restrained for movement in a fluid filled tube of nan-magnetizable material. The purpose of the time delay feature is to avoid tripping of the armature as a result of slight overloads in the supply current. High overload currents act to substantially instantaneously attract the armature even before the plunger moves from its "out" to its "in" position.
Although circuit breakers of the general type described above have been utilized in the trade for many years with satisfactory results, present day equipment has been adapted to withstand substantially higher momentary inrush currents than equipment of the past. Furthermore, on startup of an electrical load, relatively high inrush currents are experienced for a very brief time causing premature tripping of the armature even before the plunger has moved from its out to its in position. Therefor, the object of the present invention is to provide an economically feasible modification to a conventional electromagnetic circuit breaker so as to accommodate higher momentary inrush currents without such instantaneous tripping of the armature.